The high-dollar barrage spurred once wary Democrats to launch their own big money machine — which ended up nearly matching the GOP’s overall spending and actually put more ads on the air than the Republicans.

Mega-donors dragged out the primary and exposed Mitt Romney to damaging attacks on Bain Capital and social issues, some of the same attacks that Obama used to take him down. And outside money weakened the traditional party establishment’s hold on the process and propped up insurgent Senate candidates who lost on Tuesday.

Both sides are already planning their next big money pushes, but some of the top GOP donors are asking whether they wasted their time and money in 2012, and are considering changing their ways somewhat going forward.

“I have not been a big fan of ads from Day One,” said Foster Friess, whose money kept Rick Santorum alive in the Republican primary, adding he was planning to shift his cash from television ads to grass-roots organizing.

“I’m sort of burned out right now — just how much effort and resources I put into it — but I think it’s money well spent because it’s part of the process. And you don’t always win,” the retired mutual fund pioneer told POLITICO.

Karl Rove’s Crossroads outfit is holding a phone call for its big donors Thursday to sum up the race, said Stan Hubbard, a Minnesota media mogul and mega-donor. “Obviously, somebody made a mistake and didn’t do things right. There’s no question about that,” he said.

Romney and his allies spent $1.2 billion on the race, compared with $1 billion spent by Obama and his allies, according to a POLITICO analysis of records on Federal Election Commission data and public statements. Nearly 40 percent of Team Romney’s spending came from super PACs and other unlimited outside money groups, compared with about 12 percent for Team Obama.

But the final tally for 2012 might never be known because some of the biggest spending outfits, particularly on the right, are nonprofit groups that are allowed to keep their donors and many details of their spending a secret from the public.

With slim prospects for any significant clampdown on campaign cash from Congress, the Internal Revenue Service or the courts, a $2 billion presidential campaign could seem quaint in 2016, especially with competitive primaries on both sides.

“Crossroads, which you helped found, spent what, $325 million, and we’ve ended up with the same president, the same Democratic majority in the Senate and the same Republican majority in the House. Was it worth it?” Chris Wallace asked Rove on Fox News about an hour after most media had called the race for Obama.

“Yeah,” Rove responded. “Look, if groups like Crossroads were not active, this race would have been over a long time ago. President Obama came out of the box on May 15 with $215 million of advertising over a 2½-month period, designed to demonize Mitt Romney.”